Controller and method for output ripple reduction of a jittering frequency switched mode power supply

ABSTRACT

A controller for an SMPS is disclosed. The controller applies a frequency jitter to the SMPS to reduce Electromagnetic Interference (EMI) and/or audible noise. A second input variable is multiplied by a correlated jitter signal, in order to compensate the output power for the frequency jitter. A corresponding method is also disclosed. Since the jitter compensation occurs within the controller, the method is particularly suitable for controllers operating under different control modes for different output powers (or other output criteria). The multiplicative compensation is applicable across a wide range of converter types.

FIELD OF THE INVENTION

This invention relates to controllers for switch mode power supplies(SMPS) and in particular to SMPS with jittering on the switchingfrequency. It further relates to methods of controlling such switchedmode power supplies.

BACKGROUND OF THE INVENTION

It is well known that electromagnetic interference is an important issuefor SMPS. For an SMPS operating at a fixed frequency, the EMI spectrumhas strong peaks at discrete frequencies, which may exceed allowable EMIlimits.

Another issue with SMPS converters is audio noise. The audio signals canresult from low power modes or burst modes which are used to providehigh efficiency operation, and which have switching frequencies in theaudible region.

A known solution to this problem, particularly for EMI interference, isto vary the operating frequency around the specific fixed frequency, inorder to “smear out” the peaks. This is known as jittering thefrequency: jittering is well known, and a variety of different types ofsignal can be used, provided generally only that the jittering resultsin an average frequency which is that of the underlying specificfrequency. Non-limiting examples of possible jitter functions are sinefunctions and triangular functions, (either symmetric or saw-tooth).

The use of frequency jittering can also spread audible noise over awider frequency spectrum, so that it will be perceived by the human earas a lower level of noise. Especially when random jitter is applied withsufficient amplitude of the jitter, this is interpreted as white noisethat is less irritating than sound including dominant frequencies.

An example of the application of frequency jitter is disclosed in U.S.Pat. No. 6,249,849 B1, Balakrishnan, et al., “Frequency JitteringControl for Varying the Switching Frequency of a Power Supply. In U.S.Pat. No. 6,249,876 B1, the frequency jitter is applied by using anoscillator that determines the operating frequency and the oscillatoroutput is then used as clock for a counter. The counter value isconverted to an analog value in the current domain and the current isfed back to the charge and discharge current of the oscillator.

Introducing frequency jitter can result in more complex control. Forinstance, when the frequency is adapted by a main regulation loop it isdifficult to apply frequency jitter because the bandwidth of theregulation loop should be much lower than the modulation frequency ofthe jitter to prevent that the applied frequency jitter is compensatedby the regulation loop. This low bandwidth requirement of the controlloop can conflict with a fast response on a load step.

It would be desirable to be able to compensate for frequency jitter. Itis known from United States Patent Application publicationUS2005/253636, in which jittering is applied to the control of a flybackconverter by adapting the frequency by a jittering signal, to add a copyof the jitter signal to the primary peak current in order to compensatefor the change in output power introduced by the frequency jitter.

In United States Patent Application publication US2005/253636, first andsecond jitter currents are used. The switching frequency increaseswhenever the first jitter current increases. The impedance of anattenuator is decreased whenever the second jitter current increases.This reduces the on-time of a PWM output signal which compensates forthe frequency change and keeps the output power constant. The jittercontrol is based on adding current to the charge and discharge currentsof the oscillator of the PWM controller.

United States patent application publication US2006/031689 discloses asimilar summation method, in which a copy of the jitter signal is addedto the primary peak current. In this case the jitter signal is a digitalpattern, rather than an analog signal.

However, such summation methods suffer the disadvantage that summationis an absolute compensation, rather than a relative compensation. Thus,the methods are well suited to a single operating frequency, but lesssuited to variable operating points, such as variable frequencies oroutput power.

There thus remains a need for a method of jitter compensation, whichdoes not suffer from the above disadvantages to the same extent.

SUMMARY OF THE INVENTION

It is an object of the present invention, to provide a controller for anSMPS, and a method of operating an SMPS, which does not suffer from theabove disadvantages to the same extent.

According to a first aspect of the invention, there is provided acontroller for a switched mode power supply and being operable tocontrol the switched mode power supply in dependence on a frequencyinput signal and in dependence on a further input signal, comprising:

a frequency jitter unit for multiplying the frequency input signal by afrequency jitter signal to derive a modified frequency input signal tobe applied to the switched mode power supply, and

a further jitter unit for multiplying the further input signal by afurther jitter signal to derive a modified further input signal to beapplied to the switched mode power supply,

wherein the further jitter signal is correlated to the frequency jittersignal such that the jitter applied by the frequency jitter unit and thejitter applied by the further jitter unit vary in proportion with eachother.

By modifying the frequency input signal and the further input signal bymultiplication, and before application to the switched mode powersupply, the jitter function can be used for different control schemes(for example PWM control, PFM control or in combined PWM and PFMcontrol). The multiplications can be performed independent of the actualvalues of the frequency input signal or the further input signal.

In embodiments, the correlation is such that variation in an outputpower of the switched mode power supply due to the frequency jittersignal is cancelled by the further jitter signal.

Thus, the desired combination of the jittering of the frequency and asecond variable (such as peak current), is then such that for everyoperating point, the delivered output power is the same as if there wereno jittering, because the jitter components cancel each other. Thismeans the controller of the invention can be applied to different modesof operation, such as frequency control or peak current control, withthe jittering process handled by an independent unit which provides thecorrect settings for the jittering function for different operatingpoints. This independent unit ensures that the effect of the jitteringis not visible at the delivered output power of the power supply.

Particularly in the case of known, defined relationships between theoutput power and the input frequency and further input, it may betrivial to determine the correlation. For instance, in embodiments, thecontroller is configured to control a flyback converter in discontinuouscontrol mode by means of primary peak current control, the other inputis primary peak current, and the further jitter signal is opposite insign and half the magnitude of the frequency jitter signal. Thiscorrelation between the frequency jitter signal and the further jittersignal is a direct consequence of the relationship between the outputpower (P), the switching frequency (Fswitch), and the primary peakcurrent (Ipeak):

$\begin{matrix}{P = {\frac{1}{2}*L*{Ipeak}^{2}*{Fswitch}}} & (1)\end{matrix}$

In embodiments the controller is configured to control a buckboostconverter or a flyback converter, and further configured to operate theconverter in discontinuous conduction mode. Such embodiments areparticular advantageous, in that there is a fixed 2:1 ratio at which theoutput power has no ripple.

In embodiments the frequency jitter signal has the form of one of thegroup comprising a triangular function, a saw-tooth function and a sinefunction. Thus the frequency jitter signal has a gradual variation overone period with a rising slope and a falling slope, although other formsof jitter functions are not excluded from the invention, A triangularfunction is particular convenient, since it results in a uniformdistribution of the jittered frequency.

In embodiments, at least one of a frequency of the frequency jittersignal is less the 1 kHz, and a depth of the frequency jitter signal isbetween 0.9% and 33%. By limiting the jitter frequency (that is to say,the inverse of the period over which the switching frequency varies dueto the jitter) to less than 1 kHz, audible noise, which could be adistraction to an end user, is reduced or minimised.

In embodiments, the controller has a yet further input different to thefurther input and being adapted to operate according to a first controlmode at a first output power, and a second, different, control mode at asecond output power, further comprising a yet further jitter unit formultiplying the yet further input by a yet further jitter signal,wherein yet the further jitter signal is correlated to the frequencyjitter signal. In such embodiments, the controller may be adapted toswitch between two control modes at, for instance, a specific operatingfrequency. Conventional methods of providing jitter may result in rapidcycling beyond the specific frequency and back, due to added jitterwhich changes the output power, and thus rapid cycling between twocontrol modes; this phenomenon is termed “bouncing”. In embodiments ofthe invention, jitter compensation may be carried out within the controlloop, and are thus embodiments of the invention may prevent or avoidsuch bouncing between the two control modes of operation.

In embodiments, the controller comprises at least a part of anintegrated circuit.

According to another aspect of the present invention, there is provideda method of controlling a switched mode power supply in dependence on afrequency input and in dependence on a further input, comprising:

multiplying the frequency input by a frequency jitter signal; and

multiplying the further input by a further jitter signal,

wherein the further jitter signal is correlated to the frequency jittersignal such that the jitter applied by the frequency jitter signal andthe jitter applied by the further jitter signal vary in proportion witheach other.

The correlation may be such that variation in an output power of theswitched mode power supply due to the frequency jitter signal iscancelled by the further jitter signal. In embodiments the frequencyjitter signal has the form of either a triangular function or a sinefunction. Furthermore, in embodiments, a jitter frequency may be lessthan 1 kHz, and additionally or in the alternative, a jitter depth maybe between 0.9% and 33%.

In embodiments, the controller operates according to a first controlmode at a first output power, and a second, different, control mode at asecond output power, wherein, whilst the controller is operating in thesecond control mode, multiplying a yet further input, different to thefurther input, by a yet further jitter signal, and wherein yet thefurther jitter signal is correlated to the frequency jitter signal.

These and other aspects of the invention will be apparent from, andelucidated with reference to, the embodiments described hereinafter.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will be described, by way of example only,with reference to the drawings, in which

FIG. 1 shows a SMPS control schema without jitter;

FIG. 2 shows a SMPS control schema with jitter according to embodimentsof the invention;

FIG. 3 shows a control schema for a flyback converter with primary peakcurrent and frequency control and including jitter;

FIG. 4 shows the control schema of FIG. 3, and including a fixed primarypeak current and frequency regulation loop;

FIG. 5 shows simulated results of operating the control schema FIG. 4;

FIG. 6 shows a control schema for a SMPS having fixed frequency andprimary peak current regulation, including the regulation loop;

FIG. 7 shows simulated results of operating the control schema of FIG.6;

FIG. 8 shows two control inputs for a SMPS having two modes of control;

FIG. 9 shows a control schema for the flyback converter of FIG. 3,including both mode detection and control in the regulation loop;

FIG. 10 shows a block circuit diagram for an embodiment of the inventionapplied to a flyback converter with peak primary current control;

FIG. 11 shows the simulated results of operating the control schema ofFIG. 10; and

FIG. 12 shows graphs to explain a control method in which random jitteris applied to make audible noise less disturbing.

It should be noted that the Figures are diagrammatic and not drawn toscale. Relative dimensions and proportions of parts of these Figureshave been shown exaggerated or reduced in size, for the sake of clarityand convenience in the drawings. The same reference signs are generallyused to refer to corresponding or similar feature in modified anddifferent embodiments

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the invention based in part on the realisation that, formany converter types, such as, without limitation, flyback, boost, buck,and boost-buck, there is at least one other way, besides frequencycontrol, to regulate the output power. Then, in contrast to the knowntechnique of US2005/253636 in which a copy of the jitter function isadded to the output in order to compensate of the jitter, the inventorshave appreciated that a predetermined feed-forward action may beimplemented, wherein the effect of the frequency jitter is cancelled byapplying an appropriate, correlated, jittering function to one of theother input variables. As a result of the correlated jittering, thesystem acts, as far as output power is concerned, as if there were nojitter in the first place.

FIG. 1 shows a SMPS control schema 1 without jitter. The schema has ninput variables (Xa, Xb, . . . Xn) including one input variable Xfdefining the switching frequency, together with an output variable Out.A general equation for a converter defined by n input variables (Xa, Xb,. . . Xn) and including one input variable Xf defining the switchingfrequency, together with an output variable Out, operating in a certainoperating point is:Out=F(Xa,Xb, . . . Xn,Xf)  (2)so that perturbations result in:

$\begin{matrix}{{dOut} = {{\frac{\delta\; P}{\delta\;{Xa}} \cdot {dXa}} + {\frac{\delta\; P}{\delta\;{Xb}} \cdot {dXb}} + {\ldots\mspace{14mu}{\frac{\delta\; P}{\delta\;{Xn}} \cdot {dXn}}} + {\frac{\delta\; P}{\delta\;{Xf}} \cdot {dXf}}}} & (3)\end{matrix}$It is possible to maintain Out=constant by varying one or more variablesat the same time based on their sensitivity to the output.

As a non-limiting example, consider the case of a flyback converter.Equation 2 in this case takes the formP=F(L,Ipeak,Fswitch)  (4)where P is the output power, Ipeak is the primary peak current (orrather, more accurately, a signal that defines the actual primary peakcurrent), and Fswitch is the operating frequency (or rather, moreaccurately, a signal that defines the actual switching frequency). For aflyback converter, equation 4 can be rewritten:

$\begin{matrix}{P = {\frac{1}{2}*L*{Ipeak}^{2}*{Fswitch}}} & (5)\end{matrix}$

Since the sensitivity of Ipeak and Fswitch to the output power P are:

$\begin{matrix}{{\frac{\delta\; P}{\delta\;{Ipeak}} = {L*{Ipeak}*{Fswitch}}}\;{And}} & (6) \\{\frac{\delta\; P}{\delta\;{Fswitch}} = {\frac{1}{2}*L*{Ipeak}^{2}}} & (7)\end{matrix}$respectively, then:

$\begin{matrix}{{dP} = {{\frac{\delta\; P}{\delta\;{Ipeak}} \cdot {dIpeak}} + {\frac{\delta\; P}{\delta\;{Fswitch}} \cdot {dFswitch}}}} & (8)\end{matrix}$So, keeping output power P constant results in:

$\begin{matrix}{{\frac{\delta\; P}{\delta\;{Ipeak}} \cdot {dIpeak}} = {{- \frac{\delta\; P}{\delta\;{Fswitch}}} \cdot {dFswitch}}} & (9)\end{matrix}$Thus, equivalently:

$\begin{matrix}{{{dIpeak} = {{dFswitch}*{\left( {- \frac{\delta\; P}{\delta\;{Fswitch}}} \right)/\frac{\delta\; P}{\delta\;{Ipeak}}}}}{{Since}\text{:}}\begin{matrix}{{\left( {- \frac{\delta\; P}{\delta\;{Fswitch}}} \right)/\frac{\delta\; P}{\delta\;{Ipeak}}} = {- \frac{\left( {{1/2}*L*{Ipeak}^{2}} \right)}{\left( {L*{Ipeak}*{Fswitch}} \right)}}} & {{~~~~~~~~~~~~~~~~~~~~~~~~~~~}(11)} \\{= {{{- 1}/2}*{Ipeak}*{Fswitch}}} & {{~~~~~~~~~~~~~~~~~~~~~~~~~~~}(12)}\end{matrix}} & (10)\end{matrix}$This results in:dIpeak=−dFswitch*½*Ipeak/Fswitch  (13)ordIpeak/Ipeak=(−½)*dFswitch/Fswitch  (14)

Thus, in this case the correlation between the frequency jitter andprimary current jitter is −2:1. That is to say, for every 1% jitter onfrequency, ½% jitter on the primary peak current, is required forcompensation.

In general, this 2 to 1 (or square law) correlation is valid forconverters that charge their energy reservoir only from the input anddischarge completely this reservoir only via the output. This is thecase for flyback converters and buck-boost converters operating indiscontinuous mode. For operation in continuous conduction mode thecorrelation increases above 2 to 1, but is not constant and depends onthe operation point of the converter. For buck converters the chargingcurrent flows via the output and for boost converter the dischargecurrent flows via the input: buck and boost converters have acorrelation higher than 2 to 1 in discontinuous mode and the ratiodepends respectively on the absolute output voltage and input voltage.The ratio further increases for continuous conduction mode for buck andboost converters.

FIG. 2 shows a SMPS control schema with jitter according to embodimentsof the invention; the figure shows the same basic SMPS schema 1 as inFIG. 1, together with a jitter Block 21. The jitter block 21 appliesjitter to the frequency defining variable Xf at a frequency jitter unit22, and jitter to a further variable Xc at a further jitter unit 23.That is, ‘Xf’ controls the operating frequency of the converter, therebycontrolling the output, while ‘Xc’ determines a second variable of theSMPS that controls the output (for example primary peak current oron-time or even input voltage in a flyback). In this way it is possibleto choose the variation for the combination of the at least 2 variablesin such a way that the output does not change.

In frequency jitter unit 22, a variation on the operating frequency isapplied by a multiplier function wherein a signal Xf is multiplied by afactor “1+a”, where A varies as a function of time. In the same way, atfurther jitter unit 23 a signal ‘Xc’ is multiplied by “1+b”, where Bvaries as a function of time. If the variation for the combination ofthe at least 2 signals is chosen in the correct way, then there is ajittering at the operating frequency, without a net effect on the outputpower. In FIG. 2 a multiplier is used where an input variable ismultiplied by “1+a” or “1+b”. In the case of a flyback converter, theratio A:B to provide zero effect on the output is, as determined above,−2:1.

Advantageously over the prior art summation arrangement, which maintainsan absolute jitter with frequency variation, the multiplier arrangementaccording to embodiments of the invention provides that the jitteringitself is relative (rather than absolute) in frequency controlledsystems: for example a 8 kHz jitter variation at a 50 kHz switchingfrequency reduces to a 4 kHz jitter variation at 25 kHz switchingfrequency; in each case, the level of smearing out of harmonics, at thestart of the EMI band at 150 kHz, is 24 kHz: this provides an optimalsolution for smearing out EMI noise. In the prior art, the compensationis optimal only for a small range of operating frequencies around apredetermined operating point, while the multiplier solution with 2multipliers is optimal for every operating point, provided only that theinput variable has a proportional relation with the output variable.

A preferred minimum depth of frequency jitter may be determined by thefollowing: considering a bandwidth of average measuring receiver of 9kHz and a typical maximum expected switching frequency of 500 kHz: theminimum depth of frequency jitter is from −0.9% to +0.9%. Conversely, apreferred maximum depth of frequency jitter arises when the smearing outof the first spectral component (first harmonic) touches the smearingout of the second spectral component (second harmonic): the differencebetween spectral components is the first harmonic frequency and maximumdepth of frequency jitter is therefore from −33% to +33%.

The application of jitter can be used to reduce EMI interference asexplained above, but it can also (or instead) be used to reduce audiblenoise. The applied frequency jitter spreads the spectrum of the audiblesignals over a wider area so that the human ear interprets this as noiseinstead of one irritating frequency. This noise is less irritating andis interpreted as a lower volume level.

The skilled person will immediately appreciate that jitter on othercontrol variables than Ipeak may be used to correlate with the frequencyjitter. Further, provided that the correlation is such that thefrequency jittering is cancelled (which can be readily determined fromthe appropriate form of equation 3 setting dOut to zero), jitter on aplurality of other control variables, at the same time, may be used.However, to realise convenient control, it will generally be the casethat, for any individual control mode, it will be preferable to applyjitter on just one input variable other than frequency, to achieve thecompensation or cancellation.

FIG. 3 shows a control schema 30 for a flyback converter in DCMoperation mode with primary peak current and frequency control andincluding jitter. The figure is substantially similar to FIG. 2, with aflyback converter 31 having primary current (Ipeak) control Xip, andfrequency control Xf. The converter includes a main control unit 32configured to drive switch 33, which connects and disconnects inductor34 across the supply via sense resistor 35 (used to feed back theprimary current to the main control unit 32); the output circuitprovides output voltage Vout and comprises a rectifying diode 36 andsmoothing capacitor 37.

Equation 5 above applies to this arrangement:

$\begin{matrix}{P = {\frac{1}{2}*L*{Ipeak}^{2}*{Fswitch}}} & (5)\end{matrix}$and the ratio in sensitivity of output power to Ipeak and to Fswitchresults in output power remaining constant provided that equation 14holds:dIpeak/Ipeak=(−½)*dFswitch/Fswitch  (14)

In this case, the effect of the frequency on the output power will becancelled by providing correlated jitter signals a and a/2 to Xf and Xiprespectively.

For example, where a sine function is used for jitter, such asα=0.04*sin(1256·t),  (15)then the outputs Xf_and Xip_i from the frequency jitter unit 22 and theprimary peak current jitter unit 23 respectively, are:Xf _(—) i=[1+0.04*sin(1256·t)]*Xf,  (16)andXip _(—) i=[1−0.02*sin(1256·t)]*Xip,  (17)

According to embodiments of the invention, frequency jitter may beapplied to an SMPS without its being directly observable in the outputpower—not only the average output power, but also the momentary outputpower. Thus, the problem of a ripple voltage and ripple current at theoutput of the supply due to the addition of jitter may be eliminated.This is advantageous for the control, since the control inclusive ofjitter block may be considered as if no jitter were present at all. Forexample, the signals Fswitch and Ipeak in FIG. 3 fulfill equation 4 asif there were no jitter, although the actual Ipeak_jitter andFswitch_jitter vary with the jittering signal A.

FIG. 4 shows the control schema of FIG. 3, and including a primary peakcurrent and frequency regulation loop. Of course, the skilled personwill appreciate that this is one of many alternative regulation loopsthat fall within the scope of the invention and in particular any kindof regulation mechanism that can be added to this jittering system ofFIGS. 1-3 in order to get the desired behaviour of the output variable,while only a residual effect of the jitter on output variable and inputvariables occurs.

The main regulation loop compares, at 41, Vout with a reference voltageVref, resulting in an error signal Verror, which is filtered infiltering block 42. The loop thereby regulates the input ‘Xf’ in orderto regulate Vout to the desired value in combination with a fixed valuefor the primary peak current setting (Xip). Due to the applied jittermechanism according to the invention, the jitter frequency is notpresent in the inputs Xf, Xip and Vout when the regulation loop isclosed, while both the actual operating frequency setting and Peakcurrent setting (Xf_j, Xip_j) of the flyback converter include jitter.

FIG. 5 shows simulated results of operating the control schema FIG. 4.The figure shows Fswitch 51 along with the jittered versionFswitch_jitter at 52, which follows Fswitch, but with a slow sine-curvejitter. The curves Ipeak 53 and Ipeak_jitter 54 are also shown—Ipeakbeing constant, and Ipeak_jitter having a sine curve, opposite to thaton Fswitch_jitter superposed thereon. The bottom curve 55 shows Vout.

It can be seen that, after a ramp-up phase for Fswitch and Vout, thefrequency jitter applied to Fswitch resulting in Fswitch_jitter, iscancelled by the correlated jitter applied to Ipeak. As a result theoutput voltage Vout is entirely free from jitter.

FIG. 6 shows a control schema for a SMPS according to another embodimentof the invention. In this case, the SMPS has a fixed frequency andprimary peak current regulation. That is to say the input Xf to thefrequency jitter unit 22 is constant at a fixed level; conversely, theprimary peak current input Xip, which is input into the further jitterunit 23, is modified by the error signal derived from filter block 42.

FIG. 7 shows simulated results of operating the control schema of FIG.6; in this figure, the switching frequency Fswitch 72 is constant, andmodified by the sine wave jitter function to result in the jitteredswitching frequency Fswitch_jitter 71; conversely the controller is bymeans of the peak current (which thus varies rapidly according to theon-off cycle of the SMPS) Ipeak 73, to which are correlated jitterfunction is applied to result in a jittered peak current Ipeak_jitter74. As a result of the correlation between the two jitter functions, theoutput voltage Vout is free from jitter. It should be noted thatalthough a sine-wave jitter signal is shown in this figure, it may bepreferable to use a triangular jitter signal since this provides thebest “averaging out” of the EMI signal

The jitter compensation results in an output voltage Vout which isindependent of the input's jitter function. As a result, the inventionis particularly suited for use in conjunction with control schemas whichinclude a changeover of mode. For example, with flyback converters it ispossible to use the Ipeak control method described above with referenceto FIG. 6 and the frequency control method described above withreference to FIG. 4 in a single application, depending on the momentaryoutput power level. FIG. 8 shows two control inputs for a SMPS having tomodes of control: on the x-axis (abscissa) is plotted the output power,and on the y-axis (abscissa) is plotted at curve 81 the frequencycontrol input Xf, and at curve 82 the peak primary current control inputXip. At lower power levels less than a changeover power level 83, thepeak primary current control input Xip remains constant, whilst thefrequency control input Xf increases linearly with power. At powerlevels higher than the changeover power level 83 the frequency controlinput Xf remains constant, whilst the peak primary current control inputXip increases with increasing power. Due to the quadratic relationshipbetween the power and the primary peak current, this latter increasetakes the general form of a square root function.

FIG. 9 shows a control schema for the flyback converter of FIG. 3,including both mode detection and control in the regulation loop; thatis to say, configured to operate the above method. The arrangement isgenerally similar to that shown in FIG. 6, with the exception that inthis case, the error signal from the comparison between the out and therest is fed to a mode detection and control unit 91: outputs from themode detection and control unit 91 are fed to both the frequency jitterunit 22 and the other jitter control unit 23.

FIG. 10 shows a block circuit diagram for an embodiment of the inventionapplied to a flyback converter with peak primary current control. Suchan arrangement is implemented by NXP semiconductors in the integratedcircuit number TEA1721, configured to operate a converter indiscontinuous conduction mode, and varying either the primary peakcurrent or switching frequency to control the output power.

In this arrangement, a current controlled oscillator 102 is fed with acurrent Ioscj, which comprises the current lose supplied from a controlunit 104, which is multiplied by a jitter signal (1+a) at 103 such thatIoscj=(1+a)*Iosc  (18).As shown in the lower left call-out of the diagram, in this embodimentof the jitter function that may be a symmetrical triangular function,which transitions smoothly by a fixed slope positive or negative df/dtbetween the ±−8% peak excursions from the average value. However, othersjitter functions, such as without limitation saw-tooth functions or sinecurves, may be used instead of the symmetrical triangular function shownand discussed above. The current controlled oscillator produces anoscillating signal Foscj in dependence on Ioscj; the oscillating signalFoscj is input to a logic and driver units 108. Further, the controlsignal Ipeak from the control unit 104, is multiplied at 105 by acorrelated jitter function to result in the jittered version Ipeakj. Inthis embodiment, the correlated jitter function is (1−a/2). The jitteredversion Ipeakj is converted into a voltage signal Vpeakj by means senseresistor 106. This is fed to a comparator 107, to provide an errorsignal which also is input to the logic and driver unit 108. The logicand driver unit 108 drives the switch 110 of the flyback converter. Theswitch current is monitored by means of sense resistor 109 to provide ofthe second input to comparator 107. Feedback is supplied to the controlunit 104 by means of a sampled feedback signal Vfbs, which is comparedat a comparator 111 to a reference value Vref. The flyback converterswitch 110 is thus controlled by the logic and driver unit 108, withjitter on either or both of the primary current control signal of thefrequency control signal.

The jitter is added to the control current for the oscillator frequency(yielding Ioscj) and to the control current for primary peak current(Ipeakj), controlling the current in the primary winding of thetransformer in such a way that constant output power is achieved. Whenthe jitter increases Ipeakj than Foscj (Fosc=Fsw, where Fsw=switchingfrequency) is decreased and when the jitter decreases Ipeakj than Foscjis increased. The switching frequency is varied in a range from −8% to+8% without any change in power for each operation mode.

FIG. 11 shows the simulated results of operating the control schema ofFIG. 10. Starting at the top curve the figure shows the output power1101, the output voltage Vout 1102, the primary current 1103, theun-jittered and jittered peak current Ipeak and Ipeakj, 1104 and 1105respectively, the un-jittered and jittered oscillator current lose andIoscj 1106 and 1107 respectively, and the jitter function A at 1108. Inthis case, the jitter function is a sine wave. As the figure clearlyshows, following a ramp-up period, the output voltage and output currentare independent of the jitter signal. That is to say, by including thecorrelated signal, the frequency jitter has been cancelled.

Some of the examples above are designed specifically to reduce EMIinterference. For reducing audible noise most effectively, the jittercan be applied randomly.

The top part of FIG. 12 shows a simplified representation of themagnetizing current in a transformer, when a fixed operating frequencyis applied. The lower part of FIG. 12 shows a simplified representationof the magnetizing current in a transformer, when a fixed operatingfrequency is applied with random variation of the frequency where themomentary frequency F is varied randomly according to:F=(1+a·mod)×F _(nom).

F_(nom) is the nominal frequency and a is a random number between −1 and1 having a new random value for each next switching cycle. The value“mod” is the modulation index being a value between 0 and 1. In FIG. 12,mod is equal to 0.75.

The use of a random or pseudo-random jitter converts the noise to whitenoise.

The invention can be applied to AC to DC converters (for example withpower factor correction), compact fluorescent lighting controllers andLED driver applications.

From reading the present disclosure, other variations and modificationswill be apparent to the skilled person. Such variations andmodifications may involve equivalent and other features which arealready known in the art of switched mode power suppliers, and which maybe used instead of, or in addition to, features already describedherein.

Although the appended claims are directed to particular combinations offeatures, it should be understood that the scope of the disclosure ofthe present invention also includes any novel feature or any novelcombination of features disclosed herein either explicitly or implicitlyor any generalisation thereof, whether or not it relates to the sameinvention as presently claimed in any claim and whether or not itmitigates any or all of the same technical problems as does the presentinvention.

Features which are described in the context of separate embodiments mayalso be provided in combination in a single embodiment. Conversely,various features which are, for brevity, described in the context of asingle embodiment, may also be provided separately or in any suitablesub-combination.

The applicant hereby gives notice that new claims may be formulated tosuch features and/or combinations of such features during theprosecution of the present application or of any further applicationderived therefrom.

For the sake of completeness it is also stated that the term“comprising” does not exclude other elements or steps, the term “a” or“an” does not exclude a plurality, a single processor or other unit mayfulfill the functions of several means recited in the claims andreference signs in the claims shall not be construed as limiting thescope of the claims.

The invention claimed is:
 1. A controller for a switched mode powersupply and being operable to control the switched mode power supply independence on a frequency input signal and in dependence on a furtherinput signal, comprising: a frequency jitter unit for multiplying thefrequency input signal by a frequency jitter signal to derive a modifiedfrequency input signal to be applied to the switched mode power supply,and a further jitter unit for multiplying the further input signal by afurther jitter signal to derive a modified further input signal to beapplied to the switched mode power supply, wherein the further jittersignal is correlated to the frequency jitter signal such that the jitterapplied by the frequency jitter unit and the jitter applied by thefurther jitter unit vary in proportion with each other.
 2. Thecontroller according to claim 1, wherein the correlation is such thatvariation in an output power of the switched mode power supply due tothe frequency jitter signal is cancelled by the variation in the outputpower of the switched mode power supply due to the further jittersignal.
 3. The controller as claimed in claim 1, wherein the frequencyjitter signal has the form of one of the group comprising a triangularfunction, a saw-tooth function and a sine function.
 4. The controller asclaimed in claim 1, wherein a frequency of the frequency jitter signalis less than 1 kHz, and/or a depth of the frequency jitter signal isbetween ±0.9% and ±33%.
 5. The controller as claimed in claim 1,configured to control a flyback converter mode by means of primary peakcurrent control, wherein the further input is primary peak current, andthe further jitter signal is opposite in sign and half the magnitude ofthe frequency jitter signal.
 6. The controller as claimed in claim 1configured to control a buckboost converter or a flyback converter, andfurther configured to operate the converter in discontinuous conductionmode.
 7. The controller as claimed in claim 1, wherein the frequencyjitter signal comprises a jitter that varies randomly or pseudo randomlyover time.
 8. The controller as claimed in claim 1, having a yet furtherinput signal different to the further input signal and being adapted tooperate according to a first control mode at a first output power, and asecond, different, control mode at a second output power, furthercomprising a yet further jitter unit for multiplying the yet furtherinput signal by a yet further jitter signal, wherein yet the furtherjitter signal is correlated to the frequency jitter signal such that thejitter applied by the yet further jitter unit and the jitter applied bythe frequency jitter unit vary in proportion with each other.
 9. Anintegrated circuit comprising the controller as claimed in claim
 1. 10.A switched mode power supply comprising a controller as claimed in claim1 and the switched mode power supply controlled by the controller.
 11. Amethod of controlling a switched mode power supply in dependence on afrequency input and in dependence on a further input, comprising:multiplying the frequency input by a frequency jitter signal; andmultiplying the further input by a further jitter signal, wherein thefurther jitter signal is correlated to the frequency jitter signal suchthat the jitter applied by the frequency jitter signal and the jitterapplied by the further jitter signal vary in proportion with each other.12. The method of claim 11, wherein the correlation is such thatvariation in an output power of the switched mode power supply due tothe frequency jitter signal is cancelled by the further jitter signal.13. The method of claim 11, wherein the frequency jitter signal has theform of one of the group comprising a triangular function, a saw-toothfunction and a sine function.
 14. The method of claim 11, wherein atleast one of a jitter frequency is less than 1 kHz, and a jitter depthis between 0.9% and 33%.
 15. The method of claim 11, wherein thecontroller operates according to a first control mode at a first outputpower, and a second, different, control mode at a second output power,the method comprising, whilst the controller is operating in the secondcontrol mode, multiplying a yet further input, different to the furtherinput, by a yet further jitter signal, and wherein the yet furtherjitter signal is correlated to the frequency jitter signal such that thejitter applied by the yet further jitter unit and the jitter applied bythe frequency jitter unit vary in proportion with each other.